In recent years, air pollution created by internal combustion engines such as used in automobiles has become of great concern. This has led to the passage of air pollution control laws which require automobile manufacturers to reduce the emission of harmful materials from automotive engines. Typical of the pollutants to be controlled are hydrocarbon emission, carbon monoxide emission and nitrous oxide emission.
Automotive manufacturers have introduced the catalytic converter to control these emissions. However, the effective operation of the catalytic converter is dependent upon maintaining a proper level of oxygen in the engine exhaust gas resent in the catalytic converter. This has led the automotive industry to develop an accurate, yet inexpensive oxygen sensor for use in the catalytic converter.
One type of oxygen sensor that has been developed for use with catalytic converters includes a ceramic sensor, typically formed of zirconium oxide. The sensor can be formed into many shapes, but is typically in the shape of a tube or cone with an outer electrode on the outer surface of the sensor and inner electrode on the interior surface of the sensor. The outer surface of the sensor is exposed to the exhaust gas. The interior of the sensor is exposed to the atmosphere. Charged oxygen ions can migrate through the sensor ceramic at a rate depending upon the relative content of oxygen in the exhaust gas and the atmosphere. The migration of the charged oxygen ions generates a potential between the inner and outer electrodes which is proportional to the oxygen content in the exhaust gas.
Various designs have been proposed for such an oxygen sensor. U.S. Pat. No. 3,844,920, issued to Burgett, et al. on Oct. 29, 1974, discloses an air-fuel ratio sensor. A zirconia element 20 is employed with an inner electrode 33 and an outer electrode 35. The inner electrode 33 is electrically connected through a metal gasket 42 to a body member 37. A centerbore 45 in the body member 37 permits air into the hollow zirconia element 20. A metal gasket 28 is positioned between shoulder 27 on element 20 and shoulder 14 on shell member 12.
U.S. Pat. No. 3,847,778, issued to Riddel on Nov. 12, 1974, discloses an air-fuel ratio sensor. No apparent seal is provided to isolate the inner electrode on zirconia cell 30 from the exhaust gases.
U.S. Pat. No. 3,960,693, issued to Weyl, et al. on June 1, 1976, discloses a device for electrochemically measuring the concentration of oxygen in combustion gases. The device disclosed includes a tubular member 11 of an ion-conducting solid electrolyte material, such as zirconium dioxide. A terminal member 16 having an annular bead 17 is in electrical contact with the interior layer 14 on tubular member 11. An electrically conductive glass melt 19 and a metallic annular member 20 secure the member 16 within the interior of the member 11. The member 16 is hollow and has air inlet openings 22 which permit the atmosphere to communicate with the interior of the tubular member 11. The end of the terminal member 16 distant from tubular member 11 is provided with a plug-in terminal 21. Rings 31 and 34 are positioned between the housing 23 and tubular member 11 for a seal.
U.S. Pat. No. 4,132,615, issued to Linder, et al. on Jan. 2, 1979, discloses an internal combustion engine exhaust gas oxygen sensor and catalyzer combination. The patent discloses a contact flag 10 with a bore 13 to provide ambient air into the hollow interior of an electrolyte tube 3. The contact flag 10 is mechanically secured in the end portion 4 of the tube 3 and is in electrical contact with the inner electrode 6 formed in the tube 3. A press connection 9 provides for mechanical holding of the contact flag 10 and, likewise, for electrical connection thereof with the inner electrode 6. A steel ring 11 protects the end of the melted end press 9, and is used to compress the electrically conductive connection 9 so that the melt 9 will provide good electrical contact between the electrode 6 and contact flag 10. Press 9 and steel ring 12 are positioned between tube 3 and socket 7.
U.S. Pat. No. 4,169,778, issued to Mann, et al. on Oct. 2, 1979, discloses a heated solid electrolyte oxygen sensor. The sensor includes a solid electrolyte tube 12 and an electrode terminal member 14. Member 14 includes a central tubular portion 14a and a circumferential flange 14b at its lower end for contact with a sealing ring 30. The sealing ring 30, in turn, contacts the inner electrode 58 on the electrolyte tube 12. Conductive coating 58b can be a stripe across the end face of the electrolyte tube 12 or be a continuous circumferential coating. The electrode terminal member 14 includes an aperture 74 which permits air entering the annular passage 68 to contact the inner electrode 58. Exhaust gas leaking past sealing ring 32 could reach aperture 74 as washer 20 is only a flat mica washer. U.S. Pat. No. 4,175,019 and No. 4,178,222 disclose a similar design.
U.S. Pat. No. 4,222,840 issued to Murphy, et al on Sept. 16, 1980, shows no apparent way for air to enter the cavity above upper electrode 18 of disc 12.
None of these designs has proven totally satisfactory. Ideally, the oxygen sensor should provide significant sealing protection to prevent exhaust gases from entering the interior zone of the ceramic sensing element to prevent spurious oxygen content measurements. Naturally, when an engine is started, the sensor will rapidly rise from ambient temperature to its normal operating temperature of between 700.degree. and 1000.degree. F. The sealing must be effective during this rapid temperature rise and thereafter. Therefore, any sealing design must account for the significant thermal expansion encountered in the sensor as the rapid temperature rise to operating conditions takes place, yet perform the function inexpensively.